The long-term goal of the planned investigations is to provide a clearer understanding at the cellular and molecular levels of the roles of neuropeptides as chemical messengers in the nervous system. Studies of the roles of neuropeptides are facilitated by using experimental preparations where both the peptide-secreting neurons and the targets for their actions are well-defined can be investigated from animal to animal at the cell membrane level under nearly physiological conditions. We are using as a main example the neuroendocrine bag cell neurons, located in the abdominal ganglion of the marine mollusk Aplysia. Recent studies indicate that the bag cells function as a multitransmitter system that synthesizes and releases several neuropeptides derived from a common precursor protein. Two of these peptides, egg laying hormone (ELH) and alpha-bag cell peptide, are putative neurotransmitters that modulate activity in identified target neurons for minutes or hours. Two others, beta- and gamma-bag cell peptide we have recently found may mediate other effects. Using combined electrophysiological and biochemical techniques we plan to continue a detailed cellular analysis of the bag cell system in order to unequivocally establish the roles of the neuropeptides as transmitters and to obtain a better understanding of the mechanisms by which the peptides act. In particular, using voltage-clamp techniques we will continue to study the cellular and subcellular mechanisms of action of ELH and alpha-bag cell peptide on identified target neurons. We also plan to study the roles of alpha-, beta- and gamma-BCP as autoexcitatory transmitters which may generate the repetitive impulse discharge of the bag cells, to study conjoint action of peptides on individual neurons, to identify additional bag cell transmitters and to continue to define by biochemical methods the end-products of precursor processing in the bag cells. These studies should result in a clearer understanding of fundamental mechanisms of chemical signaling between neurons and of neuromodulatory systems such as the bag cells that utilize multiple peptides as neurotransmitters.